The present invention relates to integrated circuits and to methods for manufacturing them.
Where a high quality thin dielectric is required in integrated circuits based on silicon chemistry, it is general practice today to grow rather than deposit the oxide. Thin gate dielectrics (250 .ANG. or thinner) are being used with great frequency in today's VLSI circuits. However, the continued scaling of lateral circuit dimensions also tends to require thinner dielectrics, and very thin grown oxides are susceptible to increased defect density if process conditions are not exactly correct.
One particular limitation of the conventional growth methods arises when it is desired to form a thin oxide over heavily doped silicon, and particularly over heavily boron doped single crystal silicon. (Oxides grown over heavily boron-doped silicon will tend to contain a large enough fraction of boron to significantly degrade their electrical characteristics.)
This is a particular problem in applications where it is important to minimize the voltage dependence of the capacitance. This is commonly done by maximizing the dopant concentration in a single crystal semiconductor, in order to reduce the depletion width (and the variation of depletion width with applied potential) beneath the insulator. However, as noted, this can run into difficulties when the underlying semiconductor is heavily boron doped.
Silicon nitride can be grown or deposited as a thin dielectric, but is also subject to reliability problems, including pinholes, leakage, and instability in the CV (capacitance as a function of voltage) characteristics. See P. Pan et al., 14 J. Electronic Materials 617 (1985), which is hereby incorporated by reference. Grown oxynitrides have also been tested as thin gate dielectrics, and show some promise; but in some cases it is desirable to have an oxide dielectric, due to subsequent processing conditions.
Thus, it has been recognized that it would be desirable to have a method for depositing thin oxide films with quality close to that of grown oxide films. However, no such process has heretofore been successfully developed. Many chemistries for depositing silicon oxides have been reported in the literature, including tetraethylorthosilane (TEOS), silane plus nitrous oxide, dichlorosilane plus nitrous oxide, and silane plus oxygen. Some processes for depositing silicon oxides by CVD have been found to be unsuitable for contamination reasons. For example, chemical vapor deposition of oxides from tetraethylorthosilane (TEOS) tends to provide oxides which contain levels of mobile ion impurities (such as sodium) which are high enough to preclude their use as thin oxides for capacitors.
Among the deposition chemistries previously explored was dichlorosilane (SiH.sub.2 Cl.sub.2. also abbreviated as DCS) plus nitrous oxide, using [N.sub.2 O]/[DCS] ratios of 1 or greater. See W. A. Brown and T. I. Kamins. 22 Solid State Technology 51 (1979): R. S. Rosler, 20 Solid State Technology 63 (1977): K. Watanabe et al., 128 J. Electrochemical Soc. 2630 (1981): and M. E. Zvanut et al., 14 J. Electronic Materials 343 (1985); all of which are hereby incorporated by reference.
The present invention teaches that, when using this chemistry, a [N.sub.2 O]/[DCS] ratio of less than one should be used, instead of the ratios (typically 4 or 5, and always greater than 1) taught by the known prior art.
As is well known in the art, deposited films normally need high temperature anneals to achieve optimal electrical characteristics (i.e. if the electrical characteristics are to approach those of a thermal gate oxide). (See the Zvanut et al. article cited above.) The present invention provides results in the as-deposited state which are superior to those available in as-deposited films in the prior art, but it must be understood that these as-deposited characteristics will be further improved by the subsequent annealing step.
Oxides deposited according to the present invention provide electrical characteristics very close to those of grown oxides. In particular, oxides deposited according to the present invention provide higher and more uniform breakdown voltages than are possible with previously known methods for chemical vapor deposition of oxides. Moreover, oxides deposited according to the present invention tend to have a very low concentration of mobile ion impurities, so that their electrical characteristics are appropriately stable. Moreover, oxides deposited according to the present invention tend to have a low defect density and a low density of pinholes. Moreover, oxides deposited according to the present invention tend to have a high breakdown voltage. Moreover, oxides deposited according to the present invention tend to have good conformality (substantially better than that of many grown oxides).
Thus, one advantage of the present invention is that it provides a method whereby a deposited oxide can be used in any location where grown oxide is normally used.
A further advantage of the present invention is that it permits use of deposited oxides in processing stages where grown oxides would normally have been required, and thus reduces the integral .sqroot.Dt to which the structure being fabricated is exposed.
A further advantage of the present invention is that it provides a method for formation of very high quality thin oxides which is relatively insensitive to the composition of the underlying structure. For example, the present invention can be used to deposit a dielectric which will serve, in some locations in the circuit, as the gate oxide for transistors whose gates are in the second polysilicon level, and, in other parts of the circuit, will serve as the capacitor dielectric for capacitors whose lower plate is on the first polysilicon level and whose upper plate is on the second polysilicon level.
A further advantage of the present invention is that it permits fabrication of trench capacitors with consistent leakage and breakdown characteristics and with capacitance nearly as high as that permitted by the area of the cell with as thin a dielectric as permitted by leakage and breakdown constraints.
A further advantage of the present invention is that it permits fabrication of capacitors over heavily p-type semiconductor substrates. A further advantage of the present invention is that it permits fabrication of dielectrics having quality comparable to that of a grown oxide on silicon, over substrates which are not silicon at all. For example, the present invention can be used to provide high quality capacitors over III-V semiconductors (or over II-VI compounds, or over Group IV semiconductors other than pure silicon, or over ferroelectrics, or over any other material which can withstand the deposition temperatures required).
A further embodiment of the present invention uses the high quality oxide deposition process described to form sidewall oxide filaments next to MOS gates. A further advantage of this class of embodiments of the present invention is that the gate sidewall oxide thus formed has a very low density of contaminant species.
A further embodiment of the present invention uses the high quality oxide deposition process described to form sidewall oxide passivation on the floating gate structures of a floating-gate memory transistor. A further advantage of this class of embodiments of the present invention is that the floating gate is passivated with less loss of electrically effective area due to bird's-beaking, and with less integrated .sqroot.Dt, than would be required for passifying this sidewall by oxidation.
In general, the present invention teaches that high quality silicon oxides can be chemical vapor deposited using a source gas mixture which includes a halogenated silane component plus an oxygen source, wherein the atomic ratio of silicon to oxygen in the feed gases is greater than one.
Nitrous oxide is preferable to other oxygen sources since it is a relatively mild oxidizer, and thus less likely to release active radicals away from the heated surface and lead to free space nucleation. Similarly, dichlorosilane is preferred to silane or monochlorosilane, since it is a less active reagent. Optionally, trichlorosilane could be substituted for dichlorosilane, although this is at present believed to be less preferable.
A further advantage of the method of the present invention is that, for a given thin film thickness, this thickness can be formed by a chemical vapor deposition according to the present invention more rapidly than it could be grown under the growth conditions required to produce a high quality oxide film.
Another problem with thermal oxidation is that it is not entirely conformal; for example, in growing an oxide on the interior surface of a silicon trench, the oxide will tend to be thinner at the corners of the trench. This is just the opposite of what would be desirable, since the geometrically induced electric field enhancement means that the oxide will be under higher stress at the corners, just where it is thinnest.
A further advantage of the present invention is that the oxide deposited by the methods of the present invention is highly conformal.
Another problem with conventional grown oxide methods arises when the grown oxide is formed on a non-planar surface. In particular, when trench capacitor cells are used (such as in dRAM cells having trench storage capacitors), the oxide interface will necessarily be grown on silicon having more than one orientation. Unfortunately, the rate of oxide growth is sensitive to the crystal orientation of the silicon, and thus the resulting structure will tend to be thicker in one direction than another; that is, even if the trench itself is perfectly circular in cross section, the uneven oxide thickness deposited will tend to change this to produce more of an oval opening in cross section. This means that if the thinnest part of the oxide is made thick enough to prevent breakdown or excessive leakage, the thicker parts of the oxide will be thick enough to substantially degrade the specific capacitance.
Thus, one object of the present invention is to provide a method for fabricating trench capacitors having thin and high quality dielectric of uniform thickness along the sides of the trench.
A further advantage of the present invention is that passivation of polysilicon sidewalls can be performed efficiently. For example, in the fabrication of EPROMs it is normal to grow a thin oxide layer over the first polysilicon layer, to insulate the sidewalls of the floating gate. However, this oxide growth step can lead to deformation of the polysilicon sidewalls, and this can lead to miscellaneous other problems. Using the present invention. EPROMs can be fabricated using only CVD oxides for sidewall passivation, without the requirement for the growth step. Thus, deformations are not introduced and the resulting filament problems do not occur.
Thus, the present invention enables at least the following advantages, in addition to others mentioned elsewhere:
1. Formation of high-quality films at lower temperature for less time than required for formation of the same equivalent thickness by oxidation. PA0 2. Formation of high-quality oxide films on a partially fabricated integrated circuit while subjecting the partially fabricated structure to less integrated .sqroot.Dt than would be required for formation of the same equivalent thickness by oxidation. PA0 3. A deposition method which permits a deposited oxide to be used in any location where grown oxide is normally used. PA0 4. A method, for forming high-quality oxides in any desired location, which is relatively insensitive to the composition of the underlying material. PA0 5. Fabrication of capacitors over silicon which is heavily doped with boron. PA0 6. Fabrication of capacitors over p+ silicon. PA0 7. Dielectrics having quality comparable to that of a grown oxide on silicon can be formed over substrates which are not silicon at all. PA0 8. Formation of thin dielectric films with low defect density. PA0 9. Formation of thin dielectric films with low density of pinholes. PA0 10. Formation of thin dielectric films with low mobile-ion density. PA0 11. Formation of thin dielectric films with good uniformity. PA0 12. Formation of thin dielectric films with good conformality. PA0 13. Formation of thin dielectric films with high breakdown voltage. PA0 14. Formation of thin oxide films with low stress. PA0 15. Fabrication of trench capacitors with large capacitance. PA0 16. Fabrication of trench capacitors with consistent leakage and breakdown characteristics and with large capacitance. PA0 17. Fabrication of floating-gate memory cells with minimal time at high temperature required. PA0 18. Formation of high-quality capacitors having heavily boron-doped silicon as the lower plate thereof. PA0 19. Formation of silicon integrated circuits including high-quality capacitors whose capacitance is relatively independent of applied voltage. PA0 20. Fabrication of MOS transistors having gate sidewall oxide filaments with a very low density of contaminants. PA0 21. Passivation of polysilicon sidewalls can be performed efficiently, without inducing geometric distortions. PA0 22. Passivation of floating gate sidewalls with low leakage levels and low geometric distortions.
According to the present invention there is provided: An integrated circuit including trench capacitors, comprising: a substrate: at least one trench in said substrate: a dielectric material lining at least a part of the sidewalls of said trench, said dielectric material consisting essentially of the composition SiO.sub.x Cl.sub.y, where x is in the range from 1.75 to 1.95 inclusive and y is in the range from 0.01 to 0.04 inclusive; and a conductive material in said trench, said conductive material, said dielectric material, and said substrate defining a capacitor therebetween.
According to the present invention there is also provided: A thin-film dielectric material consisting essentially of the composition SiO.sub.x Cl.sub.y, where x is in the range from 1.75 to 1.95 inclusive and y is in the range from 0.01 to 0.04 inclusive.
According to the present invention there is also provided: A method for depositing a thin film of silicon oxides, comprising the steps of: providing a substrate on which the thin film of oxides is to be deposited; heating said substrate; passing a gas flow over said substrate, said gas flow including a silicon bearing component and an oxidizer, wherein the atomic ratio of silicon to oxygen in said gas flow is greater than one.